The proposed approach to the study pluripotential stem cells in the mouse is based on recently demonstrated successful transfusion of bone marrow into normal, non-irradiated recipients and the use of a new strain of mice allowing electrophoretic determination of donor and host cells on periodic peripheral blood samples. Specifically, we propose (1) to study the regulation of the level of pluripotential stem cells by following changes in the excess of these cells we found in transfused normal animals; (2) to study whether the failure of bone marrow to restore irradiated animals after several transfers is due to transfusion itself or increased numbers of divisions. We shall subject transfused animals to repeated marrow depletion and regeneration. If and only if transfusion is crucial, the transfused cells should disappear faster than the host cells; (3) by culturing committed stem cells, we wish to determine whether an excess of pluripotential stem cells established by marrow transfusion can influence intermediate stem cells and possibly mature cells. The studies should contribute to our understanding of the behavior of transfused marrow cells. The results may be of clinical importance, particularly if present attempts to prevent adverse reactions of marrow transfusions in man are successful.